US 8066419 B2
The present invention relates to lighting device (10). The lighting device comprises a light guide plate (12), and at least one array of light emitting diodes (LEDs) (14), which LEDs are accommodated in holes arranged in the light guide plate, wherein each hole has: at least two side facets (18) through which light from the LEDs is to be laterally coupled into the light guide plate, and at least one corner (20) formed by two converging side facets of the at least two side facets. This promotes TIR at adjacent light sources and therefore diminishes losses due to absorption or scattering at adjacent holes.
1. A lighting device, comprising:
a light guide plate defining a plurality of holes, and
at least one array of light emitting diodes (LEDs) at least partially disposed in the holes, wherein
each hole comprises at least two converging side facets through which light from an LED disposed in said hole is to be laterally coupled into the light guide plate, the side faces forming at least one corner pointing at an adjacent hole.
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This application is a national stage application under 35 U.S.C. §371 of International Application No. PCT/IB2007/054104 filed on Oct. 9, 2007, which claims priority to European Application No. 06122321.0, filed on Oct. 16, 2006, and European Application No. 07100359.4 filed on Nov. 1, 2006, both incorporated herein by reference.
The present invention relates to lighting device comprising a light guide plate and a plurality of light emitting diodes.
Progress in the brightness, lumen efficacy and affordability of solid state light sources such as light emitting diodes (LEDs) enables new lighting applications that are no longer restricted to niche markets. LEDs offer several advantages over traditional light sources, such as long lifetime, low operating voltage, instant on, etc. For these and other reasons, LEDs are becoming more and more suited for making lamps for several applications such as color variable lamps, spotlights, LCD backlighting, architectural lighting, stage lighting, etc.
For many lighting applications, the light of a single LED is not sufficient, and light of multiple LEDs needs to be combined to form a light source. One solution is to mix light of multiple LEDs in a light guide, before the light leaves the lighting device. An example of such a solution is disclosed in the document “LED Backlight designs using Luxeon high flux light source solutions” by Lumileds, Seattle 2004, http://www.lumileds.com/pdfs/Luxeon_light_source_solutions.pdf. A backlight based on side-emitting LEDs described in this document is schematically illustrated in
However, when in such a solution the in-coupling holes 104 are closely spaced, it may occur that light from one LED 108 a leaves the light guide 102 through a neighboring hole 104 and gets absorbed or scattered at the LED 108 b inside this hole. This is illustrated by exemplary ray trace 112 in
It is an object of the present invention to overcome or at least alleviate this problem, and to provide a lighting device with improved lumen efficiency.
This and other objects that will be apparent from the following description are achieved by means of a lighting device comprising a light guide plate and at least one array of light emitting diodes (LEDs), which LEDs are accommodated in holes arranged in the light guide plate, wherein each hole has: at least two side facets through which light from an LED arranged in the hole is to be laterally coupled into the light guide plate, and at least one corner formed by two converging side facets of the at least two side facets.
By means of the hole structure with a corner between two in-coupling side facets, light from a LED arranged in the hole may be split up in sub-beams, each of which is more or less perpendicular to its originating side facet, allowing control of the direction of the light to prevent light from entering adjacent holes. As a result, losses due to absorption or scattering at adjacent light sources in the lighting device can be diminished, and the luminous efficiency of the lighting device can be increased. To this end, preferably, the holes are arranged such that the sub-beams will reflect off the side facets of an adjacent hole, and consequently not enter the adjacent hole.
For instance, in one embodiment, a corner of the at least one corner of one hole points towards an adjacent hole. That is, preferably, the extension of an imaginary line between the center and the corner of the hole traverses the adjacent hole. By pointing a corner of a hole in a given direction, less light will be sent to that direction. Therefore, light coupled into the light guide plate through the in-coupling side facet of one hole generally (if ever) hits the side facet(s) of the adjacent hole at larger angles of incidence (compared to the prior art case discussed above, for example), thereby increasing the probability of total internal reflection (TIR) at the side facet of the adjacent hole so that the light does not enter that hole.
In another embodiment, the corner of the one hole and a corner of the adjacent hole are pointing towards each other. That is, the corner of the one hole is pointing not only towards the adjacent hole, but specifically towards a corner of the adjacent hole. This further ensures that light from a first hole does not enter a second neighboring hole, and vice versa, but instead is reflected by TIR.
As indicated above, a hole with “a corner” is not necessarily limited to a single corner hole. To this end, in preferred embodiments, each hole has a square lateral cross-section (‘lateral’ is in relation to the plane of the light guide plate). Alternatively, a hole can be triangular, pentagonal, hexagonal, rhombic, kite-shaped, have the shape of opposing arcs, etc. Further, the shapes are not necessarily regular.
The light guide plate preferably comprises at least one out-coupling structure for coupling light out of the light guide plate. The out-coupling structure can for example be an edge of the light guide plate or a tilted mirror arranged in the light guide plate to direct light out of the light guide plate in the direction of the light guide plate normal. Alternatively, the out-coupling structures can be formed by grooves, dents or scattering material, for example. In relation to the out-coupling structure(s), the side facets of the holes are preferably aligned with the at least one out-coupling structure. Such an arrangement ensures that the light beams emanating from the holes hits the out-coupling structures from an essentially perpendicular direction, which may result in uniform and collimated light coupled out from the light guide plate. In a preferred embodiment, the light guide plate has a square shape with out-coupling edges and a linear array of square holes arranged corner to corner is aligned with the diagonal direction of the square light guide plate. In such an embodiment, the side facets of the square holes are more or less parallel to the out-coupling edges of the square light guide plate. A single LED array can be placed on the diagonal, or four LED arrays forming a square rotated about 45° in relation to the square light guide plate can be used, for example.
Instead of a linear array, the LEDs can alternatively be arranged in a circular array. In a circular array, the hole shape may be selected to promote out-coupling and/or prevent interactions between LEDs at opposite sides of the array. Also, the light guide plate may have a circular shape with an out-coupling circumference concentric with the circular LED array, to ensure that the angles of incidence towards the out-coupling structure not become too large.
The light guide plate can further have means for aiming light emitted from the LEDs towards areas of the light guide plate free from holes. The directing means can for example comprise slits or other reflecting elements. In one embodiment, wherein an LED array is arranged along a reflective edge of the light guide plate, the directing means comprises slits arranged such that light reflected by the edge is aimed towards the spaces between the holes of the array, thereby significantly decreasing the probability that light is lost at nearby light sources. In a specific embodiment, the light guide plate has the shape of a right triangle wherein one cathetus of the triangular light guide plate is formed by the reflective edge along which the LEDs are placed. The other cathetus is preferably also a reflective edge and the hypotenuse preferably comprises an out-coupling structure for coupling light out of the light guide plate, as discussed above. Such a triangular lighting device has a uniform and collimated light distribution. In another embodiment, wherein two LED arrays are arranged along each other, the directing means comprises slits arranged such that light from one array is aimed towards the spaces between the holes of the other array, thereby significantly decreasing the probability that light is lost at nearby light sources.
This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention.
A light emitting diode (LED) based lighting device according to an embodiment of the present invention will now be described with reference to
The lighting device 10 further comprises a plurality of LEDs generally designated 14 accommodated in holes generally designated 16 and arranged in the light guide plate 12. The holes 16 could be through holes (as in
Each hole 16 in the embodiment illustrated in
The holes 16 are further oriented and placed such that at least one corner 20 a of a hole 16 a is pointing towards an adjacent hole 16 b, as seen in the plane of the light guide plate. More precisely, in the embodiment illustrated in
Upon operation of the lighting device 10, light 24 in-coupled from hole 16 a through side facet 18 a into the light guide plate 12 that hits the side facets 18 b of the adjacent hole 16 b does so at larger angles of incidence compared to the prior art case illustrated in
The lighting device 10 may further comprise means (not shown) for coupling light out of the light guide plate 12, such as tilted reflective elements.
TIR in the above context presumes the that light from a holes 16 a strikes a side facet 18 b of an adjacent hole 16 b at a sufficiently large angle of incidence given the light guide plate 12 and hole 16 materials. The angle of incidence is measured with respect to the normal at the refractive boundary. For a light guide plate 12 made of glass (refractive index n of about 1.5) and holes 16 filled with air, such an angle of incidence is in the order of arcsin 1/n=42°. To this end, in the above embodiment, any light 26 exiting the hole 16 a at 42° or less with respect to the side facet normal towards the nearest side facet 18 b of hole 16 b will hit that side facet at 48° or more (>42°, safety margin of 6°) (or not hit the side facet 18 b at all), and will consequently be reflected without entering the hole 16 b. In contrast, any light exiting the hole 16 a at say 50° or more towards the side facet 18 b of hole 16 b would hit that side facet at 40° or less (<42°), and would thus enter the hole 16 b. However, when using an omnidirectional side-emitter in a square-shaped hole as above, the angle of departure cannot exceed 42°, as will be appreciated by a person skilled in the art.
Also, the angles of departure and incidence and thus the occurrence of TIR depend on the shape of the holes. Namely, the probability of TIR at an adjacent hole is generally larger for opposing acute angle corners than for opposing obtuse angle corners. To this end, the relative angle between the exit side facet 18 a and the receiving side facet 18 b of adjacent holes, which angle depends on the corner radius and the alignment of the adjacent holes, should be sufficiently large to allow TIR. In the above embodiment it has been shown that a corner radius of 90° (square-shaped holes) in a linear array is enough for TIR. Various lighting devices of the type disclosed in
The lighting device of
The lighting device 10 in
Instead of square-shaped holes 16 as in
To prevent this, the light guide plate 12 is provided with a plurality of air slits 40 arranged such that light reflected by the edge is aimed towards the spaces 42 between the holes 16, as illustrated in
Upon operation, light emitted from all four side facets of each hole 16 is directed towards the out-coupling structure 48, either directly or via at least one of the reflective cathetus edges 36 and 46, and hits the out-coupling structure 48 at an essentially right angle. This is illustrated by exemplary ray-traces 50 and it provides for a uniform and collimated light distribution.
The triangular lighting device just described can advantageously be placed in a corner of a room. Except for illumination purposes, it can also serve as a shelf, for instance for a television set. It should be noted that the triangular lighting device could be embodied without the slits 40, but such a triangular lighting device would have a somewhat degraded performance. Also, the out-coupling structure 48 could instead of being straight be curved in the longitudinal direction.
The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the embodiments illustrated in